In 2004, Professor Andre Geim et al. from the University of Manchester succeeded firstly mechanically exfoliated graphene from graphite by using “Scotch tape method” and found excellent electric conductivity of graphene through the study of a quantum hall effect by using the exfoliated graphene. In 2008, James Hone et al., researchers from Colombia University, confirmed superior strength of graphene. In the same year, Alexander Balandin et al., researchers from University of California, Riverside, measured thermal conductivity of graphene as 5,300 pW/mpK, which is double that of carbon nanotubes.
For preparation of graphene, exfoliation of graphite oxide by applying a thermal shock, exfoliation of graphite as carried out by Professor Andre Geim et al., surface growth, hydrazine reduction on graphite oxide sheet, chemical vapor deposition, and cutting nanotubes by reacting in a solution of permanganic acid and sulfuric acid have been known, but none of them go beyond laboratory preparation levels except exfoliation of graphite oxide by applying a thermal shock.
On the other hand, a method of producing expanded graphite, a shape of which is worm-like or accordion-like, by intercalating graphite flakes between graphite crystal layers by adding acids to the graphite flakes and adding a thermal shock thereto has been known since long before. Such worm-like expanded graphite is used as a filler or compression-processed to be used as a sheet having anisotropic conductivity. However, such expanded graphite resulting from loose bonding between layers of graphite is inferior to graphene in physical properties and its particle size is much bigger than graphene.
Further, during a producing process of graphite oxide, a reaction following of mixing acid and potassium permanganate is an exothermic reaction and during the mixing reaction, Mn2O7 produced by a reaction between the acid and the potassium permanganate may cause an explosion at about 55° C. or more. Thus, in this method, graphite oxide in a very small amount can be prepared only by a batch process and there is a limit on mass production of graphite oxide.
Furthermore, the graphite oxide produced as described above is dispersed in a solution containing strong acid and other ions. Therefore, in order to separate the graphite oxide, it is necessary to perform a washing process several times. By way of example, Korean Patent No. 1095584 discloses a washing method of graphite oxide with an organic solvent. However, even this method cannot sufficiently and efficiently remove ions and impurities contained in graphite oxide. Therefore, it is required to develop a process for solving the problems and allowing mass production of graphite oxide.